Air conditioning systems such as are commonly provided in office, residential, health care and other buildings frequently include components with moist or wet surfaces. A good example is the air conditioner condenser which may comprise a plurality of heat exchange fins, for example of aluminium construction, cooled by heat conductive connection with a recirculating refrigerant. Hot air to be cooled is passed over the fins, and moisture in the air condenses on the fins and drains away. In some cases the heat exchange surfaces of the condenser are cooled by a recirculating thin film of water.
The cooling surfaces are typically of a large area, are moist or wet, and provide an ideal environment for the immobilization and growth of micro-organisms which are carried by the air and which form biofilms on the surfaces. The micro-organisms include bacteria and fungi which multiply on the surface. Within months the space between adjacent heat exchange surfaces may be totally occluded by biofilm growth.
Biofilm reduces efficacy and poses major health risks. Efficacy is lost because the biofilm has poor heat transfer properties and so the thermal efficiency of the condenser is diminished. Microbial activity can also shorten the service life of a system because acidic bacterial exudates can cause serious corrosion and because the increase in biomass on, and in, the components can lessen air-flow and increase back pressure in the system, requiring maintenance cleaning after a short life in service.
The health risk arises because the presence of biofilm in turn provides a haven for the further accumulation and further growth of pathogens such as fungi, bacteria, viruses, allergens, yeasts, and moulds. Conditions for the growth of such organisms are especially favourable during periods of high humidity such as may occur when the system is off, for example at night, but also arise during normal operation. The presence of organisms is highly undesirable because they can cause illness or death in humans and animals, create odours and can damage or destroy a wide variety of materials.
Of particular concern in terms of human health and safety are endotoxins and mycotoxins which are breakdown components of fungal and bacterial cell walls and which are known human respiratory allergens. In some individuals they can trigger asthma attacks, and in all cases have been shown to cause immune response. It is currently thought that over a period of exposure this reduces the ability of the immune system to respond to antagonists and leaves the subject more prone to infection by bacteria, viruses, etc. Also of concern are fungal spores, bacterial spores and bacteria.
In health care facilities such as hospitals and nursing homes the adverse consequences of release of endotoxins and myotoxins from biomass are exacerbated because many of the patients are in a weakened condition due to their primary health care problem. Micro-organisms that would not be a major threat to a healthy person can be fatal to a patient with a diminished capacity to defend themselves from infection. Increasing attention is also being paid to other environments such as public buildings, since if pathogenic micro-organisms find their way via conditioned air or ventilation shafts, into a building they can be rapidly circulated throughout the building thereby greatly increasing the likelihood of the spread of infection and disease. The prevention of spore germination and microbial survival in air conditioning systems would help reduce the risk of illness and hypersensitivity reactions.
To date there has been no effective means of prevention of biomass growth and the only means of addressing the problem has been periodic cleaning, which is labour intensive, costly and inconvenient and which does not adequately deal with health concerns between cleaning operations. Although various coating materials have been proposed, none have been able to resist the moist conditions or remain effective in preventing biofilm formation for a useful period.
It is an object of the present invention to overcome or ameliorate at least one of the deficiencies of the prior art, or to provide a useful alternative.
Many regions of the world employ heated filtered air in buildings as a means of central heating. It will be understood that such systems contribute to the same hazards as described above for air conditioning and the invention is not limited to any particular kind of filter or airflow system.